Fix code around TracForms db handling after using tuple for 'src'.
Plugin now contributes to parent resource's context navigation,
db interface is grown by a method for reverse-lookup of forms and
the 'Form' resource is getting extended to provide more information too,
everything needed to establish infrastructure for history browsing.
With the addition of templates we need to extend i18n capabilities as well.
This is the base for later adding form reset and delete/purge functions.